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The one concept you need to know to improve traffic safety

85th Percentile for Blog
There are a few concepts that are important to understand before diving into traffic data — and perhaps no other concept is more important than the 85th Percentile.

15% of drivers cause most of the accidents on the road

The 85th Percentile is the maximum speed that 85% of drivers will not exceed on a given road if there were no stop signs, speed limits or traffic signals. It’s seen as the safest speed on the roadway because 85% of the vehicles are naturally at that speed or below it. The majority of drivers drive safely because they it is in their own self-interest to remain safe.

The concept  is important in traffic management as it is critical data in setting speed limits. Most drivers behave in a safe and reasonable manner, do not drive at excessive speeds and do not want to get into crashes. Research shows that vehicles traveling between the 50th and 85th percentile of speed have the lowest risk of crashing due to speed and drivers who exceed the 90th percentile have a significantly higher risk of crashing. Therefore many of vehicles exceeding the 85th Percentile are the ones causing the majority of the accidents.

Knowing this, the 85th Percentile is a good starting point for traffic engineers to determine where the speed limit should be set. Generally, other safety factors of the roadway must also be considered before the actual speed limit is determined. The end result is typically a speed limit that is between 10 mph lower than and the 85th percentile.

The MUTCD states that…….The most common factor considered in setting speed zone limits is the 85th percentile traffic speed.

Drivers don’t slow themselves down

Once this concept is learned and understood, it can be helpful in analyzing traffic data. For example, let’s say a new day care is built and despite a posted speed limit of 25 mph, drivers are going much faster than that past the school.

One way to understand this is by analyzing the 85th Percentile. Perhaps, the numbers come back and show the 85th Percentile is 42 mph. In that case, it’s understandable that cars are exceeding the speed limit because the road is designed for a higher speed, and this is also accommodated by the road’s environment.

To solve this issue, something is needed to slow down the vehicles such as a road diet (narrowing the lanes) or radar speed feedback sign. Drivers often need a reminder to slow them down.

How to quickly collect data on the 85th Percentile

Radar signs that collect data are a good tool for helping determine the 85th Percentile on a problematic road. Users just need to set up the sign and allow it to start collecting data. Today’s signs even automatically upload data to the cloud and run reports on the 85th Percentile without driving out to the sign. Analysis is just a couple clicks away instead of hours collecting the data and poring over an excel sheet.

There is much more to learn about analyzing traffic data but the 85th Percentile is one of the first and most important concepts needed to make our roads safe.

by Lori Miles at All Traffic Solutions


Blog, Traffic Stop Blog

5 ways to calm traffic on a busy road

Shield_Field_FrontThe question we hear most often from law enforcement officers is “how can I make my roads safer?” It’s a great question and one that’s not easily answered because, well, it depends.

It depends on the road location, whether rural or municipal, the type of road, whether arterial or local street, the speed limit, the environment and a host of other features. But that being said, we get where you’re coming from. When you call us up, you’re not really looking for an answer for your specific road, you’re asking how to assess and determine what traffic mitigation tools will work on roads similar to the one you have in mind.

For this reason, here are five ways to calm a busy road.

Put an officer on the road

Putting an officer on the road is a sure fire way to get people to slow down. Nothing puts drivers on notice more than driving past an officer holding a Lidar gun out the window of the cruiser. And while many drivers do not slow down until they see the officer, they continue to drive safely for some time going forward.

However, officers cannot be on every busy road all the time, as no police department’s budget can sustain this level of enforcement. That’s why this tool is best used selectively, where the enforcement activity will be most effective in slowing down the most egregious violators and protecting the most constituents whether they be other drivers, pedestrians or school children.

Make the road narrower

Another way to calm a busy road is by narrowing the road. Called a “road diet” narrowing a road makes drivers instinctively slow down. By making a road narrower, drivers instinctively slow down. Conversely, wide, open roads persuade drivers to increase their speed. This psychological tool is good, for example, in congested urban areas with many pedestrians and other complex driving situations. In areas where road diets are incorporated, pedestrian traffic generally increases bringing economic vitality along with it.

The challenge with a road diet is it requires support from the local or state Department of Transportation and can take a long time to accomplish. DOTs need proof beyond anecdotal evidence from law enforcement that changing the road is the best solution. Therefore, it’s best to take a collaborative approach to this solution and join with the DOT to implement this solution where it’s appropriate.

Road diets can be accomplished through extensive construction, but can also be as simple as reconfiguring the traffic movement through changes to the striping on the road.

Build speed humps or roundabouts

Speed humps and roundabouts are also tools that can be used to calm a busy road. Everyone knows the sight of an upcoming speed hump (or the uncomfortable feeling when you miss the warnings). Humps immediately slow down drivers but cannot be used on every busy road.

Many times speed bumps are requested by residents in an effort to slow down vehicles, but they quickly realize that they just move the problem to another street. Plus, the residents realize that they don’t like them either and ask for them to be removed. Some localities make the residents fund an escrow fund for speed bump removal before they will install them. Emergency vehicles and snow plows do not appreciate speed bumps/humps as they slow their response time.

Roundabouts are a great solution and save lives at intersections where they are utilized. But they can only be applied to certain situations and do nothing to calm the traffic between the intersections. They also require years of planning and budgeting by the DOT. As time goes on we will see more roundabouts due to their safety and environmental advantages.

Install automated speed enforcement cameras

Automated speed enforcement cameras are a tool law enforcement has begun to use in the last decade or so. They are effective in preventing drivers from speeding and provide very consistent enforcement based only upon speed. This lack of discretion has positive and negative consequences.

The biggest problem with cameras is that citizens do not like getting tickets from a machine and they often lead to more complaints as they are considered revenue generators rather than safety improvements. And it only takes a few cases where they are used to generate revenue or at least promote this misconception to make citizens dislike this solution. While they are great tools, budget and civil protest prevent departments from utilizing this solution. And due to the installation cost, they can only be used selectively and cannot generally be moved around so they only provide “spot” solutions where drivers know to drive the speed limit where they are installed, and do what they like in other locations.

Install a radar speed display

The final tool you can use to calm a road with many speed violators is using a radar speed display. Radar speed displays are proven to slow down drivers and make them more attentive.

They generally do not require a DOT study or infrastructure to be placed on a road. Many are portable and can be placed temporarily on just about any road so that they can be used to both calm traffic and collect data at many locations around a town.

They are also budget friendly for departments that are short staffed. While radar speed signs calm traffic they also collect data that identifies problem areas and helps a department prioritize enforcement and utilize their resources more effectively.

Additionally, radar speed displays have become web-enabled so you can track the traffic volumes and speeds in real time. There is no need to drive out to the sign to download the data as it is available on any internet-connected computer. The signs alert you to low battery and tampering, saving unnecessary trips to the sign. They can even alert you to high speed violators and congestion.

Radar speed displays are a cost effective and quickly implemented solution to traffic calming and can be a great complement to longer term engineering solutions. They can help extend limited resources at a very cost effective price.

by Lori Miles at All Traffic Solutions

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